A device of the above kind is described in U.S. Pat. No. 2,002,115, in which one of the said coaxial parts includes a contact zone on which at least one friction pad is in frictional egagement. This friction pad has a protuberance by which it is mounted on one of the turns of an associated said spring. With this arrangement, it is possible to avoid, firstly deterioration of the contact zone by, in particular, fretting of the turns of the spring, and secondly uneven wear as between some portions of the springs and others, leading to weakening or even to fracture of the springs.
That arrangement is quite satisfactory when the friction pads are initially in engagement with the contact zone, as is indeed the case in the arrangement taught by U.S. Pat. No. 2,002,115. However, when the springs are shorter, a dead zone may exist initially, to extend radially between the friction pad and the said contact zone, so that the friction pad will only come into engagement with the contact zone after some deformation of the spring has taken place radially outwardly. As a result, the friction pad may become disengaged and/or may become displaced into an inappropriate position leading to malfunctioning of the device.
Such a consideration is relevant for example in the case of a double flywheel damper, such as that described in French published patent application No. FR 2 571 461A, in which the springs are short, being arranged on a pitch circle of large diameter. In addition, they are mounted through pivoting thrust inserts on one of the coaxial parts of the damper, with a large radial gap initially existing between the spring and the contact zone.
In order to overcome this problem, the friction pad may be extended inwardly so as to give it a wedge-shaped configuration, whereby it can penetrate between two turns of the spring in the manner described in U.S. Pat. No. 4,530,673.
However, such an arrangement tends to reduce the useful length of the spring. Another solution is to cut each spring in half, and to interpose between the two halves a central insert in the manner described in U.S. Pat. 4,838,395 4,838,395 and the corresponding French published patent application No. FR 2 620 501A. Such an arrangement is again not satisfactory because it tends to reduce the performance of the damper.